Various fasteners, apparatus and methods for joining and assembling parts or subunits are known, such as welding, riveting, threaded fasteners, etc. In some instances, there is a need to cost effectively join aluminum parts, subunits, layers, etc., to other parts, subunits, layers, etc. made from other materials, such as steel (bare, coated, low carbon, high strength, ultra high strength, stainless), titanium alloys, copper alloys, magnesium, plastics, etc. and/or to another part, subunit, layer made from aluminum. Solutions for these fastening problems include mechanical fastener/rivets in combination with an adhesive and/or a barrier layer to maintain adequate joint strength while minimizing corrosion, e.g., due to the galvanic effect present at a junction of dissimilar metals. Direct welding between aluminum and other materials is not commonly employed due to intermetallics generated by the aluminum and the other materials, which negatively affect mechanical strength and corrosion resistance. In cases where direct welding is employed, it is typically some type of solid-state welding (friction, upset, ultrasonic, etc.) or brazing/soldering technology in order to minimize the intermetallics, but the mechanical performance of such joints is sometimes poor or only applicable to unique joint geometries.
In the automotive industry, the incumbent technology for joining steel to steel is resistance spot welding (RSW), due to cost and cycle time considerations (less than 3 seconds per individual joint, which may be performed robotically). In certain instances, metal structures are fabricated by joining subparts made from similar metals other than steel.
Known fasteners and methods for joining aluminum to steel, and for joining similar or the same non-ferrous metals include: through-hole riveting/fasteners, self-pierce riveting (SPR), flow drill screws (FDS or by trade name of EJOTS), friction stir spot welding/joining (FSJ), friction bit joining (FBJ), and use of adhesives. Each of these processes may be more challenging than steel-to-steel resistance spot welding (RSW). For example, when high strength aluminum (above 240 MPa) is coupled to steel using SPR or another sheet of aluminum, the aluminum can crack during the riveting process. FSJ is not widely employed in the automotive industry since joint properties (primarily peel and cross tension) are low compared to SPR. In addition, FSJ requires very precise alignment and fit-up. Similar considerations pertain to FBJ and FDS, e.g., FDS is typically applied from a single side and requires alignment with a pilot hole in the sheet to be fastened, complicating assembly and adding cost. Alternative fasteners, apparatus and methods for joining and assembling parts or subunits therefore remain desirable.